Tunable diode lasers have quickly become important for high resolution instruments for analysis of molecule spectra. Most of the applications for diode laser spectroscopy have so far been in basic research. Lead salt diode lasers emitting in the wavelength range of 3-30 .mu.m have been used to study absorption line parameters and for measurement of molecule constants. The possibility of indirectly determine different parameters such as pressure, temperature, or electrical field strength has also been successfully demonstrated. There has been a relatively small interest for the overtone bands around 1 .mu.m since these do not significantly contribute to any new information on the analyzed molecules. The main limitation for practical use of lead salt diode lasers in measurement systems has been the need for cooling of to temperatures below that of liquid nitrogen. AlGaAs and InGaAsP diode lasers emit in the 0.68-1.7 um wavelength range and can operate at temperatures up to 50 degrees centigrade A characteristic of these lasers compared to lead salt lasers is their high intensity modulation index which requires special signal analysis to obtain optimum sensitivity. A large number of molecules of interest in process control and environmental measurement applications have absorption bands in the 0.7-1.7 um range, although, these are relatively weak overtone bands. The superior performance of AlGaAs and InGaAsP lasers compared with lead salt lasers however, more than outweighs this difference and they have great potential to be used in instrumental applications.
A very interesting application is for measurement of oxygen concentration, which has many practical applications, e.g. in the medical area. Today, oxygen is usually measured using different electrochemical methods. For example, semiconductor sensors or the transformation of O.sub.2 to secondary products for later detection is frequently used. These methods have several limitations. They don't perform real-time measurements and are usually not suitable in explosive environments and moreover, they often influence the measurand since a small amount of the measured (oxygen) gas is consumed by the measurement instrument. These measurement instruments are also often sensitive to other types of gases or organic pollutions which results in a considerably limited lifetime.
From EP-Al-0 015 170 it is known that by using spectrometers with reference and measurements cells and by using optical fibers or glass prism and gas-tight boxes interference with the surrounding environment is avoided This is true concerning the elimination of unwanted contributions to the measurement result from other paths than the measurement path. However, the problem of eliminating the influence of nongasrelated transmission variations in the measurement path or optical probe is not addressed at all.
In DE-Al-3.633.93 the laser is not locked onto the frequency of the absorption line. Instead, the laser current is modulated and the absorption spectrum is studied. The absorption line is then sampled as the current is modulated to determine the gas absorption. Furthermore, the spectrum is sampled on both sides of the line to measure the nongasrelated attenuation. The signal conditioning is performed directly from the sampled spectrum which results in poor accuracy since the measurement value is the result of the subtraction of two, nearly equal, numbers. This method is the most commonly used to avoid measurement errors from attenuation caused by fog, rain etc.